suppose you've created a (complex) Interval. Unfiltered it would launch every business day at 10 AM.
Now you'd like to run some job on every first business day of the week.
The Interval you've created can be used as a driver, but since you only want it to submit your job on the first business day of the week, that's exactly what you tell the system.
(The Interval returns a list of start times. This list is then chopped into smaller lists, such that each small list is within the same week. The selected 1 just picks the first entry of each small list).
Note the big difference between "the first business day of the week" and "the first day of the week if that's a business day".
This is why a filter (Day Of Week, day 1) wouldn't work here. It would skip the submit for the week every time Monday happens to be a holiday.
In the second example the Interval just defines (e.g.) the business days, but doesn't give a specific start time.
By using a Time Of Day or a Repeat entry, you can tell the system at what time you expect a submit. In the example it is a Repeat 60.
But still, you only want to execute on the first business day of the week. Hence the checked number.
Does this help?